In electronic devices which comprise complete circuits or only individual components, for example electronic sensors, the problem exists, depending on application, that radio-frequency electromagnetic interference fields can lead to an influence on the signal which disturb the operation of the electronic device. Conversely, the electronic device itself can also be the cause of radio-frequency electromagnetic fields which are to be screened with the aid of the cap in order to avoid, for example, interfering with surrounding electronic components or the radio reception in the environment.
It is known to use at least partially conductive housings in accordance with the principle of the Faraday cage to screen off static electrical fields without further measures. In the case of radio-frequency electromagnetic fields, it is not sufficient to surround the screened components with conductors, but rather, these must be electrically connected to a reference potential in order to achieve effective screening. Otherwise, it may occur, in the worst case, that the screening itself, operating as an antenna, couples radio-frequency energy in or out and thus worsens the interference problem even more.
In the simplest case, so-called galvanic grounding of the screen to a reference potential is sufficient in this case, wherein, in the case of alternating voltages, an arbitrary potential can be selected, as a rule, in order to achieve the desired screening.
For applications in which direct galvanic grounding of the housing is inadmissible, for example for the screen housings of control devices in the automobile field, the coupling is also done capacitively by a conventional combination of a resistor with a capacitor arranged in parallel, which are constructed as discrete components with particular characteristic values for the resistance and the capacitance.
In all known solutions, this results in the problem that in the case of a housing or a cap as screen, a direct electrical connection must always be established to the reference potential which means additional expenditure. The electrical connection must also be constructed to be durable so that, for example in the automobile field, there can be no interruption of the electrical connection and thus a failure of the screen affect even after many years of use.
Particular problems in using conductive caps also exist if, for example, only one particular component is to be screened which is arranged on a circuit board or conductors of a so-called lead frame in which stamped metallic or sheet-metal elements are held by a plastic carrier. Since, as a rule, several conductors of different potentials are here arranged in the circuit board or lead frame plane, a careless arrangement of a conductive cap can rapidly lead to a risk of short circuits.